The present invention relates to disposable hygiene products and more specifically, to methods and apparatuses for processing disposable hygiene products. More specifically, the invention relates to cutting and applying segments of one web to attach to a disposable hygiene product. Various types of automatic manufacturing equipment have been developed which produce the desired results with a variety of materials and configurations.
When manufacturing hygiene products, such as baby diapers, adult diapers, disposable undergarments, incontinence devices, sanitary napkins and the like, a common method of applying discrete pieces of one web to another is by use of a slip-and-cut applicator. A slip-and-cut applicator is typically comprised of a cylindrical rotating vacuum anvil, a rotating knife roll, and a transfer device. In typical applications, an incoming web is fed at a relatively low speed along the vacuum face of the rotating anvil, which is moving at a relatively higher surface speed and upon which the incoming web is allowed to “slip”. A knife-edge, mounted on the rotating knife roll, cuts a off a segment of the incoming web against the anvil face. This knife-edge is preferably moving at a surface velocity similar to that of the anvil's surface. Once cut, the web segment is held by vacuum drawn through holes on the anvil's face as it is carried at the anvil's speed downstream to the transfer point where the web segment is transferred to the traveling web.
Typical vacuum rolls used in the prior art have rows of vacuum holes which are fed by cross-drilled ports, each being exposed to the source of vacuum by commutations, as the ports move into a zone of negative pressure in a stationary manifold. Such a configuration serves to apply vacuum sequentially to each successive row of holes.
A common problem associated with slip-and-cut applicators occurs at the point of cut. Since the web being cut is traveling at a very low velocity compared to the anvil and knife velocity (perhaps 1/20th), the engagement of the knife with infeeding web tends to induce a high tensile stress in the infeeding web. Having been placed under such a high level of stress, the infeeding web can recoil violently when the cut is finally completed, causing loss of control of the infeeding web. This “snap-back” effect increases with the thickness of the infeeding web. Thicker webs tend to prolong the duration of engagement with the knife before completion of the cut, thereby increasing the build-up of stress. This is a common process problem that is usually addressed by the provision of various shock-absorbing devices. One possible solution might have been to reduce the surface velocity of the knife, but substantially different velocities between the knife and anvil result in rapid wear of the knife edge and/or anvil face, depending on relative hardness.
Continual improvements and competitive pressures have incrementally increased the operational speeds of disposable diaper converters. As speeds increased, the mechanical integrity and operational capabilities of the applicators had to be improved accordingly. As a further complication, the complexity of the fastener tabs being attached to those products has also increased. Consumer product manufacturers are now offering tapes which are die-cut to complex profiles and which may be constructed of materials incompatible with existing applicators. For instance, a proposed fastener tab may be a die-profiled elastic textile, instead of a typical straight-cut stiff-paper and plastic type used in the past. Consequently, a manufacturer may find itself with a slip-and-cut applicator which cannot successfully apply die-cut tape segments. Existing applicators cannot successfully apply fastener whose boundaries are fully profiled, as may be desired to eliminate sharp corners, which might irritate a baby's delicate skin. This demonstrates a clear need for an improved applicator capable of applying new fastener configurations and overcoming other shortcomings of prior art applicators.
Slip-and-cut apparatus are well known for their ability to cut relatively short segments of one web and place them accurately on another, higher speed web. Certain materials, however, behave badly in these applications. The tension pulsation caused by the cutting may cause the material to snap back, losing its natural track down the moving surface of the anvil roll. This is especially common with thick webs. Other materials, such as nonwoven fabrics, may be difficult to control because they are very porous and provide little resistance to air flow to keep the material on track. Still other materials, such as certain perforated films may possess texture qualities which tend to be very unstable on the anvil surface, acting instead like a puck on an air hockey table.
These problems are further exacerbated by using materials with a very low modulus of elasticity. Here, even very low levels of vacuum at the anvil surface may cause the material to stretch with the advancing movement of the anvil. The sudden change of tension seen when the knife cuts this over-stretched web can result in severe snap-back and complete loss of position, relative to the intended centerline. Likewise, webs with very high moduli may snap back violently when the web is cut.
The prior art is quite successful when processing full-width or symmetrical webs, which are drawn uniformly forward by the sliding vacuum surface on which they are held. Attempts to process asymmetrical webs on such a surface are less successful, as the draw of the advancing vacuum pattern will act differently on parts of the web which have differing lines of tension. For instance, a die-cut ear web for a disposable diaper may have only a narrow continuous portion along one edge, with the opposite edge being more or less scalloped in shape.
It is therefore an object of this invention to provide an apparatus which can maintain control over die cut web sections of various shapes.